Periodic testing of an electrical motor is often required throughout the life of the motor, to ensure that its full rated current can be drawn and that of all its drive circuits are intact and operating correctly. Some servo actuators use electrical motors in the form of electrical drives to convert electrical energy to mechanical motion (translational or rotational); for example, an electrical drive may be connected to a ball-screw, lever arm or gimbal mechanism. In some applications, it is difficult to test electrical parts of the drive train, because of mechanical constraints. That is the case, for example, where actuation by a servo electric motor during testing is undesirable, for example because the mechanical motion that would be produced by the motor during electrical testing actuates a one-time mechanism of some kind, e.g. triggering of an irreversible latching or unlatching mechanism. Only limited tests can be carried out on such a system, if full-rated current cannot be used because of such constraints.
For example, the fins of some missiles are driven by an electromechanical system using a brushless DC-motor powered by a thermal battery. The missile fins are mechanically locked close to their zero position up to a point shortly after launch. During the missile launch sequence, prior to fin unlock, the missile is required to test activation of the actuator thermal battery. Prior-art systems have done that by driving the fins against a fin-locking mechanism, causing a large current to be drawn and thus proving that the battery is functioning correctly. Similarly, powered tests of the motor drive system may also be required during built-in test sequences, which may for example be with an external power supply. In prior-art missile designs, the locking mechanism is a pyrotechnic locking mechanism, which can withstand the driven fins during the testing. However, it would be preferable to provide a locking mechanism in the form of a simple shear-pin design, which would be less expensive and more reliable; however, were a shear-pin to be used, correct functioning of the thermal battery activation and the motor drive system could not be proved by driving the motors against the mechanism, as that would shear the locking pin. Rather, the currents used would be much less than the full rated current, to reduce the torque provided to a level low enough to avoid shearing the pin.
There therefore exists a need for a method and apparatus for operating an electrical drive system that enables testing of its drive circuits at full rated current without actuating any mechanism arranged to be actuated by the electric motor during normal (i.e. non-test) use.
In addition to testing applications, there are other situations in which it would be advantageous to be able to operate an electrical drive system without producing torque on the motor.